Similar to the "fives" rule on seats. However, with fives, you can only save your seat for five minutes. Calling squaters means that you can save your seat unless you sit down somewhere else. The moment your ass touches another seat, squaters is ended, and the first seat becomes available.

Bob: "shit my phone is ringing, but i dont want to lose my seat..."
gets up to answer phone, and someone steals his seat
Jack:"He should have called squaters"

2. One who hovers over as if to guard. Sometimes a sexual partner from what you intended as a one night stand, or that you dumped and won't take a hint, can become a squater and soon become a cockblocker.

Dude, you said your cousin was just gonna be here for a couple of days. He's become a squater and has been here two weeks now.

I thought I would just fuck her once, but now she keeps squating on me at the club and is becoming a real cockblock.